Current contra-rotation open fan (CROF) propeller systems for CROF aircraft engines require some degree of “cropping” of the blades of the downstream propeller in order to reduce takeoff community noise due to natural contraction of the stream tube beyond the upstream propeller. Current legal noise limits and local airport noise regulations may require cropping of the blades by an amount sufficient to reduce the diameter of the downstream propeller by twenty percent or more to comply with the limits and regulations.
Cropping, however, decreases the effective span, and thus lift for a given pitch of the downstream propeller, and requires an increase in pitch or re-design of the propeller airfoil to recover lost thrust to meet take-off thrust requirements. In some cases, it is not possible to meet the un-cropped thrust and aspects of a propeller re-design may further worsen fuel consumption and noise. The increased pitch, however, may result in increased drag and a degradation of off-design (take-off, climb, descent, reverse are all off-design) performance that can also increase wake-related self-noise of the cropped propeller.
The unacceptable noise levels encountered during takeoff by an aircraft having one or more CROF engines is driven by wake interaction and Blade Vortex Interaction (BVI) noise. BVI noise is caused by the fact that tip vortices created by the upstream propeller tend to decay in a radial inward direction toward the downstream propeller as the stream tube contracts (the contraction angles are a function of flight Mach number and aircraft angle) in such a way that the vortices are “chopped” (interacted with) by the downstream propeller at generally the most highly loaded region of the downstream propeller blades. As the downstream propeller chops these vortices, inherent blade-passage tone levels increase and new interaction tones are created. In addition, un-steady blade stress is aggravated, which may cause a need for propeller reinforcement, and additional engine vibration reinforcement/treatment resulting in increased weight and cost. Often, a thicker propeller airfoil than would be preferred for optimum performance is required for reinforcing strength, which increases wave drag that degrades propeller performance and creates noise.
The noise penalty relative to a downstream propeller that is “cropped” may be up to 6EPN (Effective Perceived Noise) dB (cumulative to the three current certification measurement point rules). While the tips of the downstream propeller blades are not always literally “cropped” (i.e., cut), a downstream propeller having a shorter diameter than the upstream propeller is generally referred to by those skilled in the art as being “cropped”.
Reducing the diameter of the downstream propeller of a CROF propeller system of a CROF aircraft engine holds to the same aerodynamic principles as reducing the effective wing-span of the aircraft, and results in a direct reduction in aerodynamic efficiency, which for a CROF engine can be as much as five percent cruise SFC (Specific Fuel Consumption). The typical airplane level Block Fuel penalty, encompassing the varying penalties of the overall mission segments (take-off, climb, cruise, descent) is typically slightly (1-2 percent) worse than the stated SFC penalty. Unfortunately, this fuel burn penalty is generally accepted for an entire mission even though the noise-sensitive portion of the takeoff segment may only last several minutes.
Furthermore, an airplane life cycle can be 20-30 years and, during this lifetime, legal noise certification standards as well as local airport regulations typically increase in stringency. In order to avoid costly modifications early in the airplane and engine product life-cycles, these factors can drive even greater aggressiveness in engine performance compromises to ensure the current vehicle design can meet possible future noise requirements at the time the customer takes delivery.
One proposed approach to solving the CROF noise problem is to drastically alter the aircraft general arrangement and configuration to shield/contain the noise produced by the propeller system. For an equivalent class of payload-range requirements, however, this type of solution generally involves unacceptable penalties including increased TAROC (total airplane related operating costs) due to increased airplane mass (OEW) per unit payload, degraded drag due to increased wetted area and center of gravity trim, and an additional engine specific fuel consumption penalty related to how the engine is installed in the presence of the airframe, wing, and control surfaces. In addition, these alternative options often result in an unacceptable engine-to-engine proximity for fratricide avoidance, airplane load-ability, and an engine location that becomes inaccessible for maintenance and service.
Therefore, it would be advantageous to have a method and apparatus that takes into account one or more of the issues discussed above, as well as possibly other issues.